What is a Histogram?
A histogram is a bar chart of a distribution: values grouped into bins, bar height showing frequency. Its gift is shape — one glance shows whether data is centered, skewed, spread wide, or (the telling one) has two humps, which almost always means two different processes hiding in one dataset.
In the quality toolkit it's the shape-viewer; Pareto is a specialized histogram of causes sorted by count.
Worked example
Plotting 400 concrete cylinder strengths, the histogram shows two distinct peaks. Investigation: day-shift and night-shift crews use different vibration practices. The averages looked fine all along — only the two-humped shape revealed there were two processes producing one product, and one of them needed retraining.